1. Field of the Invention:
This invention relates to an autofocusing apparatus for a photographic printer, and more particularly to an autofocusing apparatus suitable for use in a photographic printer which allows a focusing position to be set on the basis of an AF curve determined from the characteristics of a reference film carrier.
2. Description of the Related Art:
Photographic films come in a number of sizes e.g. 110 type, 135 type, Brownie type. Further, the photographic films of the 135 type come in full-size frame format and half-size frame format. To permit the use of photographic films in such varied sizes, some current printers enable variation in printing size and enlargement magnification by movement of the lens. In addition, they also enable variation in the length of an optical path that is the conjugate length of the optical path for printing. The amount of movement of the lens and of the conjugate length can be determined by an AF curve. The method of determination will be described hereinafter. FIG. 3 is a schematic of a printer. A negative film 60 held in a negative carrier 66 is illuminated by an unillustrated light source. The light thus transmitted passes through a lens 62 to illuminate a photographic paper 64, thereby enabling an image to be recorded. Here, the following equation can be established as a first lens formula: ##EQU1## where A is distance between the lens and the negative film,
S is conjugate length PA1 f is focal length of the lens. PA1 A' is distance between the standard position of the lens and the actual lens position, PA1 S.sub.P is distance between the standard position of the photographic paper and the negative film, which serves as a compensation value, PA1 S' is distance between the standard position of the photographic paper and the actual position of the photographic paper, PA1 A'm.sub., S'm.sub.1 are A' and S' when the magnification is m.sub.1 and PA1 A'm.sub.2, Sm.sub.2 are A' and S' when the magnification is m.sub.2.
Next, when a magnification m.sub.1 is specified and the focal point is adjusted by m.sub.1, the following equation is derived from Equation (1): ##EQU2##
Further, when a magnification m.sub.2 is specified and the focal point is adjusted, the following equation is derived in the same manner as Equation 2: ##EQU3## where A.sub.P is distance between the standard position of the lens and the negative film, which serves as a compensation value,
Incidentally, A' and S' are values obtained by measuring actual distances for example, by measuring the number of pulses from a pulse motor. A.sub.P, and S.sub.P are values calculated on the assumption that the above equation (1) is established when focusing is adjusted.
Since A'm.sub.1, S'm.sub.1, A'm.sub.2 and S'm.sub.2 are respectively the distances relative to each standard position their values are known. Further, since the focal length is determined by the lens, A.sub.P and S.sub.P can be determined by the above-described equations (2) and (3). As will be understood from the next equation, either A.sub.P or S.sub.P can easily be calculated, once the other is determined: ##EQU4##
Incidentally, it is essential to provide precise focusing and magnification in general printers. A practical printing operation therefore needs to obtain similarly precise focusing by inputting a desired magnification.
Namely, focusing is considered to be adequate so long as sharpness is within a certain tolerance. However, since the magnification can vary within the range able to be focused, it is necessary to provide compensation throughout the range of variation.
Accordingly, A' and S' under a predetermined magnification will be determined in accordance with the following procedure.
First the following equation can be established as a second lens formula: ##EQU5##
Here, Am and Sm are determined from the following equations in the same manner as in the above-described focusing: ##EQU6##
Since Am and Sm can be determined by the equations (6) and (7), either Am or Sm can be calculated by the following equation once the other is determined: ##EQU7##
A' and S' are determined by a simultaneous equation composed of equation (8) and the aforementioned equation (4).
If optimal A.sub.P, S.sub.P, Am and Sm are determined by repeating the above focusing operation and then stored in a memory of a control unit, A' and S', which permit focusing at a magnification m, can be easily determined by inputting the magnification.
In a method such as that described above, when the position of the negative film 60 is shifted, as seen in an imaginary line of FIG. 3, each parameter varies. In other words, a difference K-K' occurs using as a standard level a low face of the negative carrier 66 as viewed in FIG. 3, so that the preciseness of A' and S' cannot be obtained. The position of the negative film 60 varies when the negative carrier 66, adapted to hold the negative film 60, is replaced with another in accordance with the size of the negative film 60. Namely, this is caused by variations in dimension produced by machining of the negative carriers. In addition, the position of the negative film does not vary in practice. However, in case that a negative carrier is used in which glass has been incorporated in an opening where the negative film is disposed, the length of the optical path of the transmitted light is changed by the glass, leading to the same result as the position of the negative film is changed. An example of a similar cause of variations in the precise position of the negative film is where another negative carrier -- different from a negative carrier applied when an AF curve has been determined -- is employed.